The invention relates to a filtering circuit for filtering a control signal generated in a phase-locked loop. The phase-locked loop comprises a phase detector, an oscillator, and a filtering circuit connected between the phase detector and the oscillator. The phase detector determines the phase difference between an input signal, applied to the detector, and an output signal generated by the oscillator; the detector produces the control signal, the instantaneous value of which depends upon this phase difference. The filtering circuit receives this phase detecting control signal and produces a filtered control signal which is applied to the oscillator; the repetition rate of the oscillator output signal being variable as a function of the instantaneous values of the filtered control signal.
It is a known practice to provide in phase-locked loops a filtering circuit which filters a control signal supplied at the output of the phase detector stage. The filtered control signal is then supplied to an oscillator stage of the phase-locked loop. The oscillator generates an output signal, the repetition rate of which depends on the instantaneous values of the filtered control signal. Generally, a filter is utilized if there exists a substantial difference between the repetition rates of an input signal applied to the phase detector and the output signal generated by the oscillator. Accordingly, the phase-locked loop, with the aid of the filter, produces an error signal from the phase difference between the input signal and the output signal; this error signal, in the phase-locked state, is proportional to the difference between the repetition rates. Such filtering of the control signal may, however, lead to instability of the phase-locked loop; this is the case, for example, if an integrating network is utilized as a filter.